Scheduling appointments on calendar, sync between office and smartphone

Wanting to be able to schedule appointments either on my smartphone calendar or on the PC in my office, and have them instantaneously sync with each other (wirelessly), so I can schedule appointments in the field, as well as my secretary schedule them as she takes calls, and never have to reschedule due to conflicts.

Any ideas?

This might be a simple fix, haven't tried anything yet.

Also, the ability to sync calendars between Android/iOS/Windows platforms would be great as well.